1. Field of Invention
This invention relates to a helicopter rotor head wherein the blades are mounted in a fully articulated manner with three axis support freedom provided by an elastomeric type bearing, or bearings, which react the compound blade deflections and more particularly to a static and dynamic blade droop support and limit stop for use in such a rotor which isolates the droop support mechanisms from the effects of blade excursions in pitch, lead-lag, and flapping such that contact between the droop members not only avoids possible coupling effects but provides continuous full area contacting surfaces between the droop supports in reacting gravity or thrust loadings of the blades.
2. Description of the Prior Art
The droop stop art includes mechanisms that result in full area contact between two abutting surfaces which limit blade downward motions while avoiding coupling effects due to blade pitch change or blade lead-lag. It is significant to note, however, that the type of rotor head to which these prior art mechanisms are adapted is the fully articulated type having multiple and separate means to support the blades from the rotor head in pitch, lead-lag and flapping freedoms. An example of this prior art is assignee's U.S. Pat. No. 2,614,640 to Buivid, which shows mechanism long utilized by the assignee of the instant invention for Sikorsky type helicopters. The droop stop of Buivid comprises an abutment on the underside of the flapping link attached to the rotor blade, and a mating abutment which is an extension of the drag link. The effects of pitch coupling are precluded or isolated by mounting the first abutment inboard of the pitch change or stack bearings (not shown). The effects of lead-lag coupling are precluded by mounting the second abutment outboard of the drag hinge. The flapping hinge is utilized to provide the differential flapping motion between the two abutments.
As an advancement in the helicopter rotor head art, the spherical laminated elastomeric bearing rotor is substituted for the fully articulated rotor head shown in Buivid. The significant feature of this new type of configuration is that the separate support bearings of Buivid for pitch change, lead-lag, and flapping are functionally replaced by a single elastomeric unit, or units in an array. Examples of these two types of elastomeric rotor heads are shown in U.S. Pat. Nos. 3,778,189 to Ferris, and 3,782,854 to Rybicki. Various attempts have been made to provide blade droop support and coning limiting capability to the elastomeric bearing rotor configuration, but heretofore none of these have achieved the full area bearing contact of this invention for all combinations of blade excursions. While it is the basic function of the elastomeric bearing to react all blade motions in a single unit, and thus offer the potential benefit of increased reliability and maintainability of a non-lubricated member, it is this multiple reaction feature that presents the coupling difficulties. The prior art for elastomeric type rotor heads teaches droop support mechanisms which provide point or line contact only between the droop stop contacting members for all combinations of blade motions, or which result in flat bearing area contact for only one specific angle of lead-lag. Of the most pertinent of these prior art patents are the following:
Kisovec U.S. Pat. No. 3,282,350 teaches in its FIGS. 2 and 3 configuration an elastomeric bearing rotor head having full three axis articulation wherein the means to limit rotor blade droop is the contact between a flap stop on the hub and the arcuate portion of the blade. It is notable that the degree of blade droop will be a function of the amount of cyclic pitch imparted to the blade, and that full area contact between the abutting droop members will only occur at a single predetermined pitch angle.
Assignee's Ferris U.S. Pat. No. 3,778,189 provides an improvement over Kisovec wherein the pitch coupling effect is eliminated by the use of droop members shaped to form intersecting cones with their axes specifically located. Such a configuration produces line contact between the members, as pure rolling occurs between them under the various combinations of blade motions when the conical members are in contact. This patent also presents an improvement over the droop limiting mechanism shown in the Mosinskis U.S. Pat. No. 3,501,250, which comprised a hub mounted conical pad and a blade mounted roller and would produce neither pure line rolling contact nor full area bearing contact.
Assignee's U.S. Pat. No. 3,853,426 issued on Dec. 10, 1974 on improvements in "Elastomeric Helicopter Rotor Head with Dynamic and Static Blade Coning and Droop Stops," by R. Rybicki presents an improved droop stop configuration over the Ferris patent wherein both the static and the dynamic mechanisms are supported by the same mounting means and include provisions for misalignment of the main elastomeric bearing without compromise to the line contact between the droop members. Such configuration was successfully incorporated and actually reduced to practice in an H-53 six-bladed Sikorsky helicopter.
The number, size and weight of the blades and rotor are significant factors in the design of droop stop mechanisms, since the loading of the members must not be allowed to produce stresses which would cause failure of the members. Thus, for given weights and loadings, suitable contact surface must be provided on the abutting members, and suitable strength must be provided in their supporting structures. Also, the size and weight of the droop members must be optimum to avoid detrimental drag effects and overweight performance limitations.
This invention provides an improvement over the prior Rybicki application in that it teaches a droop stop configuration which provides full area contact and is adaptable to the full articulation feature of the elastomeric rotor head. For rotor heads in larger sizes and weight than the H-53, such configuration has provided these beneficial weight and stress advantages.
To specify these benefits further, it must be realized that the space envelope available for incorporation of n droop stop members for an n bladed rotor head is generally reduced as the number of blades is increased for larger helicopters, and the blade size and load to be imposed on those stop members will also be increased. Area contact droop members are generally smaller and lighter in weight than line contact members and offer less drag resistance. Thus, the configuration offered by this invention is attractive to the requirements of the stress, weights, and performance engineers. The lack of rolling contact also reduces the need for lubrication.